Heat is conventionally removed from the coolant of an internal combustion engine by passing the coolant through a radiator. One or more cooling fans are conventionally utilized to draw air across the radiator to facilitate heat removal from the coolant flowing therewithin. Cooling fans may be driven directly from an internal combustion engine or may be independently driven via a separate power source.
Cooling fans directly driven by an internal combustion engine have several disadvantages compared with cooling fans driven via separate power sources. One disadvantage is that fan speeds may be inadequate at some engine speeds which may result in inadequate or inefficient heat removal. For example, at low engine speeds, a cooling fan directly driven by an internal combustion engine may not have sufficient speed to adequately draw air through a radiator. Accordingly, at low engine speeds, the ability to adequately remove heat from engine coolant may be reduced. At high engine speeds, a cooling fan directly driven by an engine may have excessive speed, thereby drawing excessive air through a radiator. This may result in overcooling.
Furthermore, when a cooling fan is driven directly from an internal combustion engine, mechanical devices, such as belts, splined shafts, chains, and the like may be necessary to transfer power from the engine to the cooling fan. These mechanical devices may increase the complexity and expense of an internal combustion engine and may decrease the efficiency thereof.
Cooling fans driven by an electrical motor are conventionally configured to cycle on and off at predetermined coolant temperatures. Unfortunately, electrical motors serving in this capacity may have various disadvantages. One disadvantage is that an electrical motor of sufficient power to drive one or more cooling fans may place a strain on a vehicle's electrical system. Also, the physical size of some electrical fan motors may be somewhat large and, as a result, undesirable for automotive use. In addition, electrical motors for driving cooling fans may be somewhat noisy. Unfortunately, efforts to reduce electrical motor noise may add to manufacturing costs and may decrease electrical motor efficiency.
Cooling fans may also be driven by hydraulic motors. The rotational speed of a cooling fan driven by a hydraulic motor is conventionally controlled by varying the rate of flow of a working fluid being pumped to the hydraulic motor that drives the cooling fan. Typically, the rotational speed of a hydraulic motor is increased as coolant temperature increases. Conversely, the rotational speed of a hydraulic motor is decreased as coolant temperature decreases.
The rate of flow of fluid to a hydraulic motor may be controlled via a solenoid-actuated control valve. Unfortunately, conventional solenoid-actuated control valves have various limitations. For large internal combustion engines, multiple cooling fans may be required. As a result, fluid flow requirements may be rather high. For example, a pair of hydraulic motors for driving cooling fans may jointly require between 15 and 20 gallons per minute (GPM) of hydraulic fluid to adequately rotate the cooling fans.
Unfortunately, conventional solenoid-actuated control valves may not be able to sufficiently handle flow rates of this magnitude. This is because flow forces imposed on the internal components of a control valve at high flow rates are often quite large and unstable (non-linear). Unstable flow forces may result in unsatisfactory performance by a hydraulic motor. In addition, the design of many conventional solenoid-actuated control valves may lead to an increase in valve hysteresis at large flow rates.